Wow that's a really long response.  So here is a brief one.  In 2003 you create these events through wizards and we all c# code behind that K2 would generate for you.  Whereas now when you configure these wizards Microsoft Windows Workflow Foundation schedules are generated.  Now you might be saying that sounds pretty much the same but with blackpearl there are now multiple designer canvases (InfoPath, 2 in SharePoint -AJAX and Silverlight based, Visual Studio).  Whereas 2003 had 1 canvas - K2 Studio (which is being reintroduced in the 90x relese of blackpearl coming soon - it's already visible in blackpoint).

Plus in blackpearl you have something called SmartObjects.  These are multipurpose objects that you can define to collect data from multiple back end line of business systems and surface them as one object in SharePoint, ASP.net or InfoPath with no code.  Leaving the data where it lives but retrieving it when needed to make workflow decisions, or simply pushing and pulling based on workflow results.

My best advice to you is to read the K2.com site.  blackpearl has evolved K2 into a more SOA based tool - and definitely more object oriented from a BPA perspective. 

Again this is not what I would call a "great" description of the differences but it's a start from a high level.